Synergism of [C2N4H7S] cations and anionic modulators: tailoring second-order nonlinear optics and birefringence in organic–inorganic crystals

Abstract

The development of high-performance ultraviolet (UV) nonlinear optical (NLO) crystals relies on the exploration of functional units. In this work, we investigate the [C₂N₄H₇S] moiety as a promising candidate. By employing distinct non-π-conjugated anionic templates ([NH₂SO₃] and [BF₄]), we successfully synthesized two crystals: non-centrosymmetric (NCS) (C₂N₄H₇S)SO₃NH₂ and centrosymmetric (CS) (C₂N₄H₇S)BF₄. The former exhibits a significant second-harmonic generation (SHG) response 1.8 times that of KDP, while the latter shows a large birefringence of 0.500 at 546 nm. This study not only confirms the great potential of the [C₂N₄H₇S] cation for UV NLO applications but also demonstrates how anionic selection dictates the macroscopic symmetry and resultant optical properties.